Mechanical properties of yttria-stabilized tetragonal zirconia (YSTZ) bicrystals under compressive loading are investigated by atomistic simulations. Previous studies on deformation of single-crystal YSTZ showed that some specific orientations promote dislocation emission, tetragonal to monoclinic phase transformation, or both. In this work, nanograins with different orientations are selectively combined to generate bicrystals with various grain boundaries (GBs). Simulation results show that regardless of orientation of nanograins, the strength of YSTZ bicrystals is higher when the GB plane is parallel to the loading direction, and in the case of [011]/-oriented YSTZ bicrystal, the strength even exceeds that of the single crystal. Independent plastic deformation of individual grains and their interactions at the GB plane are believed to be responsible for the observed increase in strength. GB plane inhibits the volume expansion of transformed monoclinic phase and therefore serves as a source of strengthening. In contrast, YSTZ bicrystal displays softer behavior when GB plane is perpendicular to the loading direction. GB plane acts as the source of softening by initiating local amorphous phase formation.